Bacteria in the genus Yersinia cause a range of human diseases, including bubonic and pneumonic plague, infections of the intestinal lymphoid system, and lethal systemic infections. A number of virulence factors are common to pathogenic Yersinia species, including a type 3 secretion system (TTSS), and a set of secreted proteins known as the Yops and LcrV. Yersinia use the TTSS to deliver several Yop effector proteins (e.g. Yops E, H and J) into host cells. Delivery of the Yop effectors requires several additional proteins including Yops B and D, and LcrV. The effector Yops act within host cells to thwart innate immune responses through interference with signal transduction pathways. The TTSS is required for sustained replication of Yersinia in host lymphoid tissues. Two cytokines, tumor necrosis factor alpha (TNF) and interferon gamma (IFN), are essential for development of protective immune responses to Yersinia. However, Yersinia use the TTSS to counteract production of TNF and IFN in lymphoid tissues. The results of in vivo studies suggest that either LcrV or YopB are directly responsible for counteracting cytokine production. The results of in vitro experiments using cultured cells as infection models show that Yops E, H and J are directly responsible for counteracting cytokine production, and YopB and LcrV function only to mediate delivery of the effectors. Thus, there is a significant disagreement in the field concerning the mechanism used by Yersinia to counteract cytokine production by host cells. We .will utilize isogenic wild-type, Yop mutant and LcrV mutant Yersinia strains and a mouse intestinal infection model to determine how the Yops and LcrV function to counteract cytokine production in vivo. In aim 1, host cell types that interact with wild-type or Yop mutant Yersinia in tissues of mice will be identified. These studies will identify host cell types that are targets of Yops in vivo. In aim 2, levels of TNF and IFN in tissues of mice infected with wild-type, Yop mutant or LcrV mutant Yersinia will be measured. Our results will elucidate an essential mechanism of Yersinia pathogenesis.